1. Field of the Invention
The present invention pertains to the field of the coexistence of two different communication methods and apparatus, more specifically to the coexistence of a spread spectrum TDMA communication system with a GSM communication system.
2. Description of the Related Art
A growing demand for flexible, wireless communication has led to the development of a variety of techniques for allocating available communication bandwidth among a steadily increasing number of users of wireless services. One technique for allocating communication bandwidth between a base station and a group of user stations is through the use of time division multiple access (TDMA), wherein transmissions are separated in time to avoid conflicts. In a communication systems utilizing TDMA techniques, a repetitive time frame may be divided into a plurality of smaller time units, or time slots, and communication between base stations and user stations occur in assigned time slots.
Other techniques for allocating communication bandwidth include frequency division multiple access (FDMA), wherein transmissions take place at different frequencies, and code division multiple access (CDMA), wherein transmissions take place using different codes.
Variants of the above techniques include frequency division duplex (FDD) and time division duplex (TDD). As used herein, FDD refers to a technique for establishing full duplex communications having both forward and reverse links separated in frequency, and TDD refers to a technique for establishing full duplex communications having both forward and reverse links separated in time. Various systems using combinations of FDD, TDD, FDMA, CDMA, and/or TDMA have also been proposed.
A particular FDD/TDMA system in current use is the Global System for Mobile communications ("GSM"). According to GSM standards, communication between a base station and user stations takes place in a time frame which is divided into eight burst periods or time slots. In each of these time slots a different user station can communicate with the base station. No two user stations can communicate with the base station during the same time slot. GSM standards include the use of two distinct frequency bands. The base station transmits over the first frequency band, and the user stations transmit over the second frequency band. The user station transmission lags the base station transmission by several time slots to account for, among other things, propagation delays from the base station to the user station.
The GSM protocol provides for transmission and reception between remote devices and is generally suitable for communication at relatively high data rates. In addition to being a uniform standard in Europe, where it has been allocated a frequency band around the 1.9 GHz region, the GSM protocol has been tested, used and found to be robust, and there is a substantial installed base of devices and systems which utilize the GSM protocol in Europe. While the GSM protocol has not been allocated a specific bandwidth in the United States, it can be used in certain unlicensed bandwidths, and is a standard adopted by some telephone system operators seeking compatibility with user stations configured for GSM in Europe.
Certain other communication systems make use of a technology known as spread-spectrum communication, in which transmitted signals are spread across a frequency band which is wider than the bandwidth of the data being transmitted. In spread spectrum communication, a data signal is typically modulated with a pseudo-random chip code to generate a transmitted signal spread over a relatively wide bandwidth. The transmitted signal has a low spectral density and appears essentially as noise to those not knowing the chip code. Consequently, spread spectrum communication provides increased security of transmitted information and reduced interference with other sensitive radio equipment being used in the surrounding environment.
Due to the nature of the spread spectrum signal it is typically necessary at the receiver to despread the received spread spectrum signal to recover the original data. In one spread spectrum technique, for example, despreading of the spread spectrum signal is accomplished by correlating the received signal with a reference code matching the pseudo-noise code used in the transmitter to encode the data prior to transmission of the information. After initial correlation is achieved, it is generally necessary to maintain synchronization by tracking the incoming signal so as to keep it aligned with the local reference code. Spread spectrum communication has been implemented in a TDMA environment (see, e.g., U.S. Pat. No. 5,455,822 issued Oct. 3, 1995).
A general problem in wireless communication systems is that, because users of any one system may be mobile, they may leave the coverage region of their provider and enter a zone in which the provider does not provide coverage. For example, a user who has purchased a GSM based unit may travel to the United States and find that there is no communication system that will support the GSM based system. Likewise, a user who has purchased a unit that operates in a system configured for spread spectrum communication may travel out of the geographic region serviced by the system provider. While a user may solve this problem by having several different devices (e.g., handsets) for communication with different systems in different localities, switching between handsets may be cumbersome and inconvenient, as well as costly. Moreover, there is increasing consumer demand to provide lighter handsets of smaller size for easier storage and transportation.
The existence of a multiplicity of different communication systems, each of which may serve different and possibly overlapping geographic regions, has led and will continue to lead to the potential for redundant hardware deployment for base stations and supporting network connections, redundant user hardware, and interference among neighboring wireless providers. At the same time, the number of cellular and wireless users continues to grow, as do the demands for providing more sophisticated wireless data transfer services such as wireless facsimile and other similar services.
It would therefore be advantageous to provide a communication system providing increased user mobility between different communication systems without the need to purchase additional hardware. It would further be advantageous to provide a communication system allowing communication with users according to either of two protocols. It would further be advantageous to provide a means of integrating two communication protocols in an overlapping geographical region, while mitigating interference with users utilizing either of the two communication protocols. It would further by advantageous to provide a means for dynamically allocating communication resources to either of two different communication protocols according to user demand. It would further be advantageous to provide a communication system allowing compatibility within a geographic region of both a GSM protocol and an alternative protocol, thereby increasing the geographic mobility of users of both protocols.